The objetive of the research was to evaluate grain chalkness in three rice cultivars, under different conditions of climate and different sowing times. The experiment was carried out during the growing season of 1988/89, at the experimental field of the Departament of Agriculture of the Escola Superior de Agricultura Luiz de Queiroz, ESALQ/USP. The three paddy rice cultivars used were: IAC-4440, PESAGRO-104 and CNA-3771, sowed four times during the recommended sowing time for the São Paulo region, i.e. from 10/04 to 12/09/1988. The white center and the white belly of rice grams were estimated in 100 grams samples, harvested at 20% moisture content. It was conclued that there are differences in grain chalkness among the genotipes. CNA-3771 presented a higher index of white center grains than the established index by selection programs.